1.Bill no. will be as follows AB/ST/00001..2...3 etc./12-13 - pls. suggest how can i add this column in my database
2.Service Tax amount will be calculated automatically and save on the database
3.Gross amount will be calculated automatically and save on the database

I think there's a big misunderstanding of what to store in a database and how.

A database is supposed to hold unique information. When you store bills, you'll probably need something like the receiver, the amount etc. What you do not need to store is the billing number, because it's simply a "human readable" collection of various data (ID, prefix and suffix). The same goes for any derived amount like the service tax amount, the gross amount etc.

In fact, it would be wrong to store these data, because this would be redundand. You'd store the same information multiple times, which is obviously inefficient, difficult to handle and error-prone (you can easily end up with conflicting data).

So constructing the billing number has absolutely nothing to do with MySQL. This is what your application is for. Fetch the data you need and then let PHP make a string out of it.